TO clear up confusion about passivation, Jim Fritz, senior market development manager for TMR Stainless, presented a primer during the National Tank Truck Carriers' 2011 Cargo Tank Maintenance Seminar in Louisville, Kentucky.
For starters, he said stainless steel is protected from corrosion by a thin layer (passive layer) consisting primarily of chromium oxide.
He pointed out that passivation has a few different meanings:
The mechanism by which stainless steel will spontaneously form a protective film. With stainless steels, passivation occurs naturally in the presence of oxygen in the atmosphere or dissolved oxygen in aqueous environments.
A chemical treatment with an acid solution for the purpose of enhancing the formation of a protective film.
“When some people say, ‘I want my stainless steel passivated,’ they really mean they want it treated with some sort of acid that will optimize that protective film,” Fritz said. “So you really have to know what people are talking about when they say, Is my stainless steel passive?”
He said chemical passivation treatments provide several benefits:
Remove free iron, sulfides, and other foreign matter that would inhibit the formation of a good, protective passive film.
Assist in the formation of a passive film with superior corrosion resistance.
Typically performed with nitric or citric acid solutions.
Will not remove oxide scale, heat tint, or accompanying chromium-depleted zone.
“When you have chrome oxides present at the surface, not only do you have that outside layer there — which is chrome oxide — but the chrome that formed that oxide layer came from the underlying alloy, so you have a little layer of the alloy that's depleted in chrome, and ideally you'd want to not only remove the chrome oxide but also the chromium-depleted layer,” he said.
Pickling is an aggressive chemical treatment for removing oxides (de-scaling). He said it removes oxides, chromium-depleted areas, and some of the stainless-steel surface. With stainless steels, nitric-hydrofluoric acid solutions are most widely used.
“Pickling treatments result in a passive surface, and subsequent passivation treatments are not necessary,” Fritz said. “I know there are some standards out there that call for pickling followed by passivation, but that's mostly a waste of time and money if it doesn't enhance the performance of that surface anymore.”
He said confusion surrounds the question: When is natural passivation sufficient?
“The stainless-steel surface should be clean and free of contaminants such as free iron, oxides, dirt, grease, and oil,” he said. “The surface must have a smooth finish equivalent to 180 grit or finer. It's easy to know how rough your surface is. That you can measure and see. It's a lot more difficult to know if you have contamination on the surface. You don't see free iron until the free iron turns to rust. You almost, in some cases, have to rely on the chemical treatment, because you don't know if you have contamination. But if you know your shop is clean and tools are designated only for stainless steel, and you know you avoided contamination and have a smooth finish, natural passivation is a potential option.”
He said chemical passivation is recommended:
After fabrication operations such as grinding, cutting, welding, and cold forming, because fabrication frequently results in contamination.
When contaminants such as free iron are present. “If your shop fabricates carbon steel and you also do stainless steel in the same area, you're going to get free-iron contamination on that surface.”
After a corrosive attack. “If you start corrosion on your surface, you want to clean the surface very well and follow up with a chemical passivation treatment, with the goal to form a nice, passive, protective barrier with every nook and cranny of that surface.”
“You can do a weld repair, and if you do a proper post-weld cleaning, either through grinding followed by passivation or even grinding to a very smooth surface, you can get your corrosion properties up to the original base metal,” he said. “This assumes the weld procedure was done properly — proper heat inputs and proper selection of filler materials. If you do all of that properly, and post-weld cleaning, it's easy to achieve corrosion resistance that approaches the base metal.”
Small parts are dipped into a bath, and larger parts are usually sprayed. Composition of the acid, temperature, and dwell time depends on the stainless-steel grade.
Spray methods are used for large surfaces that cannot be dipped.
Pastes. Acid solutions are combined with binding agents to form a paste. It's not very effective at low temperatures (less than 50 degrees F). Avoid drying the paste — keep temperatures below 100 degrees F. “Once they dry, that stops the action.”
He said it's always better to force-dry a tank trailer after cleaning. “If you leave pools of water set inside a stainless-steel vessel for a long enough time, you're opening the doors for some really bad things to happen,” he said. “It does sort of depend on the situation. Letting it sit overnight is probably not an issue. But with a lot of stainless-steel vessels that have groomed by doing a hydrotest and leaving water sit in the tank for weeks, what can result is what we call ‘microbiologically influenced corrosion.’ If bacteria can start growing in that water, it can produce some aggressive environments. If you have any residual chemicals left over, sitting in water, if it does dry, you can concentrate some of it. So it'd be better to force-dry than allow concentrating mechanisms to happen.
“Short term, it should not affect the passivation layer unless you have a concentration and you're growing aggressive bacteria. If you just expose a stainless-steel surface to clean water, you're not doing anything to the stainless-steel surface.”